JPA JPQLThis chapter tells you about JPQL and how it works with persistence units. In this chapter, examples follow the same package hierarchy, which we used in the previous chapter as follows: Java Persistence Query language. JPQL is Java Persistence Query Language defined in JPA specification. EclipseLink/Release/2.5/JPA21. The following Java Persistence query language update statement is. With Java Persistence API 2.1 a new synchronization type. It is used to create queries against entities to store in a relational database. JPQL is developed based on SQL syntax. Entity. Manager. create. Query() API will support for querying language. Query Structure. JPQL syntax is very similar to the syntax of SQL. Having SQL like syntax is an advantage because SQL is a simple structured query language and many developers are using it in applications. SQL works directly against relational database tables, records and fields, whereas JPQL works with Java classes and instances. For example, a JPQL query can retrieve an entity object rather than field result set from database, as with SQL. ![]() The JPQL query structure as follows. Aggregate functions returns the resultant values by calculating the input values. Follow the same example employee management used in previous chapters. Here we will go through the service classes using scalar and aggregate functions of JPQL. Let us assume the jpadb. Eid. Ename. Salary. Deg. 12. 01. Gopal. Technical Manager. Manisha. 40. 00. 0Proof Reader. Masthanvali. 40. 00. Technical Writer. Satish. 30. 00. 0Technical Writer. Krishna. 30. 00. 0Technical Writer. ![]() ![]()
The Java Persistence Query Language (JPQL) is a platform-independent object-oriented query language defined as part of the Java Persistence API (JPA) specification. Kiran. 35. 00. 0Proof Reader. Create a class named Scalarand. Aggregate. Functions. These keywords are used after Where clause in a query. Create a class named Between. And. Like. Functions. List. import javax. Entity. Manager. import javax. Entity. Manager. Factory. Persistence. import javax. Query. import com. Employee. public class Between. And. Like. Functions . The usage of this clause is same as the use in SQL, but it deals with entities. Follow the Order by example. Create a class Ordering. List. import javax. Entity. Manager. import javax. Entity. Manager. Factory. Persistence. import javax. Query. import com. Employee. public class Ordering . Instead of dynamic queries, usage of named queries may improve code organization by separating the JPQL query strings from POJO. It also passes the query parameters rather than embedding literals dynamically into the query string and results in more efficient queries. First of all, add @Named. Query annotation to the Employee entity class named Employee. Entity. import javax. Generated. Value. Generation. Type. Id. import javax. Named. Query. import javax. Table. @Named. Query(query = . While transacting with the database, first it will effect on duplicate data and only when it is committed using entity manager, the changes are effected into the database. There are two ways of fetching records from the database - eager fetch and lazy fetch. Eager fetch. Fetching the whole record while finding the record using Primary Key. Lazy fetch. It checks for the availability of notifies it with primary key if it exists. Then later if you call any of the getter method of that entity then it fetches the whole. But lazy fetch is possible when you try to fetch the record for the first time. That way, a copy of the whole record is already stored in cache memory. Performance wise, lazy fetch is preferable. Querying JPA Entities with JPQL and Native SQL Developer: Java Persistence. By Yuli Vasiliev. Learn how to take advantage of the Java Persistence query language and native SQL when querying over JPA entities. Published September 2. In Java EE, the Java Persistence API (JPA) is the standard API for accessing relational databases, providing a simple and efficient way for managing the object/relational mapping (ORM) of regular Java objects (POJO) to relational data. Such Java objects are called. JPA entities or just. Like relational tables, entities are typically related to each other with relationships such as one- to- one, one- to- many, many- to- one, or many- to- many. It's fairly obvious that Java applications dealing with entities require a standard mechanism to access and navigate entity instances. The Java Persistence query language (JPQL) is specifically designed for this purpose. In this article, you will learn some interesting ways of using JPQL, as well as native SQL to query over entities utilized within a Java application. Sample Application. The code snippets discussed in the article are taken from the Java source files used in the. Looking through the sample archive, you may notice that this is a simple Web application based on the Java Servlet and Java Persistence API technologies. For simplicity, it doesn't use enterprise beans, issuing JPQL queries directly from within servlets. It doesn't mean, however, that you won't be able to utilize the JPQL queries discussed here in enterprise beans—you can define JPQL queries in any Java EE components. As you can see, it contains a set of entities related to each other with relationships of different types. Such a branchy structure is needed in order to illustrate the use of JPQL join queries discussed in the Defining JPQL Joins section later in the article. Figure 1 Relationships among the entities utilized within the sample application. For a detailed instruction on how to set up and then launch the sample app, you can refer to the readme. In fact, there are many similarities between JPQL and SQL. Both are used to access and manipulate database data, in the long run. And both use nonprocedural statements—commands recognized by a special interpreter. Furthermore, JPQL is similar to SQL in its syntax. The main difference between JPQL and SQL lies in that the former deals with JPA entities, while the latter deals directly with relational data. As a Java developer, you also maybe interested to learn that using JPQL, unlike SQL/JDBC, eliminates the need for you to use JDBC API from your Java code—the container does all this work for you behind the scenes. JPQL lets you define queries using one of the following three statements: SELECT, UPDATE, or DELETE. It's interesting to note that the Entity. Manager API interface offers methods that can also be used to perform retrieve, update and delete operations over entities. In particular, these are find, merge, and remove methods. The use of those methods, however, is typically limited to a single entity instance, unless cascading takes effect, of course. In contrast, JPQL statements do not have such a limitation—you can define bulk update and delete operations over sets of entities, and define queries returning sets of entity instances. To issue a JPQL query from within your Java code, you have to utilize appropriate methods of the Entity. Manager API and Query API, performing the following general steps: Obtain an instance of Entity. Manager, using injection or explicitly through an Entity. Manager. Factory instance. Create an instance of Query by invoking an appropriate Entity. Manager's method, such as create. Query. Set a query parameter or parameters, if any, using an appropriate Query's set. Parameter method. If needed, set the maximum number of instances to retrieve and/or specify the position of the first instance to retrieve, using the set. Max. Results and/or set. First. Result Query's methods. If needed, set a vendor- specific hint, using the set. Hint Query's method. If needed, set the flush mode for the query execution with the set. Flush. Mode Query's method, overriding the entity manager's flush mode. Execute the query using an appropriate Query's method: get. Single. Result or get. Result. List. In the case of an update or delete operation, though, you must use the execute. Update method, which returns the number of entity instances updated or deleted. The following code fragment is taken from a servlet's do. Get method that uses a JPQL query to obtain information about all the customers stored in the underlying relational table associated with the Customer entity specified in the query. The Entity. Manager's create. Query is used to create the Query instance whose get. Result. List method is then used to execute the JPQL query passed to create. Query as the parameter. As you might guess, the Query's get. Result. List method returns the result of a query as a List whose elements, in this particular example, are cast to type Customer. Note, however, that using get. Single. Result will cause an exception if you get multiple results back. Also this example illustrates the use of the Query's set. Parameter method through which you can bind an argument to a query parameter. With set. Parameter, you can bind both named and positional parameters. Here, though, you bind a named parameter. Alternatively, you might utilize the Entity. Manager's find method, which lets you retrieve a single entity instance based on the entity's id passed in as the parameter. This is what the above snippet would look like, if you need to retrieve only the value of the cust. In JPQL, you also can use a comprised select list, selecting the data only from the entity fields of interest. In that case, however, you need to create the class to which you will cast the query result. In the following section, you will see an example of a JPQL join query whose select list is comprised of the fields derived from more than one entity. In SQL, however, you normally define a join that combines records from two or more tables and/or views, including only required fields from those tables and views in the select list of the join query. In contrast, the select list of a JPQL join query typically includes a single entity or even a single entity field. The reason for this lies in the nature of the JPA technology. Once you obtain an entity instance, you can then navigate to its related instances using corresponding getter methods. This approach makes it unnecessary for you to define a query that will return all related entity instances of interest at once. For example, to obtain information about orders along with their line items in SQL, you would need to define a join query on both the purchase. Orders and order. Line. Items tables, specifying the fields from the both tables in the select list of the query. When using JPQL, however, you might define a query only over the Purchase. Order entity, and then navigate to corresponding Order. Line. Item instances using the Purchase. Order's get. Order. Line. Items method as required. In this example, you might want to define a JPQL query over the Purchase. Order and Order. Line. Item entities only if you need to filter retrieved Purchase. Order instances based on a condition or conditions applied to Order. Line. Item. The following snippet shows an example of JPQL join query in action. To better understand how the entities involved are related to each other, you can turn back to Figure 1 shown in the Sample Application section earlier in the article. This is where the SUM aggregate function may come in handy. In SQL, such a join query might look like this. SELECT SUM(p. price*l. FROM purchaseorders o JOIN orderlineitems l ON. JOIN products p ON l. What this means in practice is that you won't be able to pass p. JPQL's SUM. However, there are ways to work around this issue. In the following example, you define class Line. Item. Sum whose constructor is then used in the select list of the query, taking p. What the Line. Item. Sum constructor does is multiply p. Next, you can iterate through the Line. Item. Sum list retrieved by the query, summing the values of the Line. Item. Sum's rslt variable. The following snippet shows how all this can be implemented in code. In most cases, however, you will have to deal with queries that receive an instance or a list of instances of a certain entity. In real- world applications, though, you may need to perform some further operations on the query results. For example, you may need to update the retrieved instances and then persist them back to the database. This raises the question: are the instances being retrieved by a JPQL query ready to be further processed by the application, or some additional steps are required to make them ready for that? In particular, it would be interesting to learn in what state, concerning the current persistence context, retrieved entity instances are. If you have some experience with Java Persistence, you should know what a persistence context is. To recap, a persistence context is a set of entity instances managed by the Entity. Manager instance associated with that context. In the preceding examples, you used the Entity. Manager's create. Query method to create an instance of Query for executing a JPQL query. Actually, the Entity. Manager API includes more than twenty methods to manage the lifecycle of entity instances, control transactions, and create instances of Query whose methods are then used to execute the query specified and retrieve the query result. With respect to a persistence context, an entity instance can be in one of the following four states: new, managed, detached, or removed. Using an appropriate Entity. Manager's method, you can change the state of a certain entity instance as needed. It's interesting to note, however, that only instances in managed state are synchronized to the database, when flushing to the database occurs.
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